Numerous steps are necessary in processing materials, in particular semiconductors, where no contacts to the workpiece are allowed. Among such processes are those carried out in vacuum chambers, and where cleanliness and perfection of the sample and its surface are essential. Optical methods provide the means to probe the sample without physical contact. Optical transmittance through the sample depends on the coefficient of absorption, .alpha., as does the reflectance off the sample's surface (in those cases where the Fresnel coefficient of reflectivity does not vanish, and where the sample is quasi-transparent in the spectral range employed).
The use of a transmission method to overcome the limitations of the pyrometer has been described in some detail in U.S. Pat. No. 4,890,933 entitled TRANSMISSION METHOD TO DETERMINE AND CONTROL THE TEMPERATURE OF WAFERS OR THIN LAYERS WITH SPECIAL APPLICATION TO SEMICONDUCTORS issued on Jan. 1, 1990 to A. Amith et al and assigned to the assignee herein. In that patent, there is described a method of accurately determining the temperature of a thin layer of bandgap material without requiring contact to the layer. The method uses optical radiation through the layer to detect optical absorption by the layer. The relationship between the temperature varying bandgap energy and the resulting optical absorption characteristics provides an indication of temperature, independent of ambient temperature. Reference is also made to a co-pending patent application, Ser. No. 399,729 filed Aug. 28, 1989 which has been allowed and which is a divisional also of the application resulting in the above-noted patent and having the same title, inventors and assignee.
The present invention overcomes the severe drawbacks associated with any temperature-sensing method which requires contact to the workpiece. For all but thoroughly opaque samples, the invention also overcomes the fundamental limitations of pyrometric techniques which depend on measuring black-body (or gray-body) radiation. The latter technique measures the sample's emittance, which is extremely small unless the transmittance and the reflectance are themselves very small. This is a consequence of Kirchoff's Law. The inventive method herein discloses how to select the optimum optical wavelength to be transmitted through, or reflected off of the material sample so that the temperature of the sample can be determined accurately. The method also enables the user to investigate the bandgap effect of the material sample. As the example for this invention and the description that follows, the transmittance through the sample (GaAs) will be employed.